AN12

Application Note 12
Issue 2 January 1996
The FMMT718 Range, Features and Applications
Replacing SOT89, SOT223 and D-Pak Products with High Current
SOT23 Bipolar Transistors.
David Bradbury
Neil Chadderton
Designers of surface mount products
wishing to drive loads with currents
above a few hundred milli-amps were
previously forced to resort to using either
large through-hole products or expensive
SOT89, SOT223 and D-Pak surface mount
transistors.
voltage performance given by the
SuperSOT range, package characteristics
and chip design will not be repeated in
this note.
Features and Benefits of the
FMMT618/718 Series
Now with the introduction of the Zetex
FMMT718 and FMMT618 series of PNP
and NPN bipolar transistors, loads of up
to 6A peak, 2.5A continuous can be driven
by SOT23 packaged devices.
The key parameters of the FMMT618 and
FMMT718 series are listed in Table 1
(overleaf).
These SuperSOT devices provide many
advantages including vastly improved
circuit efficiency, component and board
space savings, and improved reliability. It
is true to say that these state of the art
devices give performance unmatched by
any other SOT23 transistor and many
larger SOT89 and SOT223 devices.
The first feature to note is that these
S O T 2 3 t r a ns i s to r s a r e c a p a b le o f
dissipating 625mW - around twice the
industry standard for SOT23 packaged
products. Achieved using a custom lead
frame, this means that for a given power
dissipation a Zetex SuperSOT chip will
run cooler than any competitors’ SOT23
devices. This gives improved reliability
and the option to reduce PCB area if
desired.
This note outlines the features, benefits
and applications of the Zetex PNP
SuperSOT series, plus some additional
NPN applications, and follows on from
application note AN11 which covered the
low voltage variant NPN types FMMT618
and FMMT619.
Since AN11 detailed how Zetex achieved
the exceptional current and saturation
Secondly, the saturation voltages are the
lowest of any SOT23 of comparable
BVCEO in the market place today and
lower than many competitors SOT89 and
SOT223 types. This translates to lower
po w e r d i s s i pati on in sw itc hing
applications, again giving improved
reliability and smaller PCB areas.
AN 12 - 1
Application Note 12
Issue 2 January 1996
Application Note 12
Issue 2 January 1996
The SuperSOT Series
POLARITY
NPN
PNP2
PNP
FMMT618
FMMT619
FMMT624
FMMT625
FMMT718
FMMT720
FMMT722
FMMT723
BVCEO
20V
50V
125V
150V
20V
40V
70V
100V
IC CONT
2.5A
2A
1A
1A
1.5A
1.5A
1.5A
1A
IC MAX
6A
6A
3A
3A
6A
4A
3A
2.5A
Midband hFE
450
450
450
450
450
450
450
450
Typical hFE
at IC
360
2A
225
2A
140
1A
45
1A
230
2A
290
1A
275
1A
250
1A
Typical
VCE(sat)
at IC
130mV
150mV
165mV
180mV
145mV
245mV
140mV
210mV
2.5A
2A
1A
A
1.5A
1.5A
1A
1A
Ptot
625mW
625mW
625mW
625mW
625mW
625mW
625mW
625mW
*Measured with device mounted on a 15 x 15 x 0.6 mm ceramic substrate.
Table 1
NPN and PNP SuperSOT Series Parametric Overview.
These features combined with high
mid-band hFE and fast switching speeds
make the series ideal for switching
applications such as DC-DC converters,
motor drivers, lamp and solenoid drivers,
display drivers, power supply line
switching, buffers etc. These transistors
will also suit linear applications, e.g. the
low series base resistance inherent in the
S u pe r S O T des ign appr oach gives
excellent low noise performance.
A f e w e x a mp l e a p p li ca t i o n s a r e
suggested below, that exploit many of
the key features of the FMMT718/618
series.
’H’-Bridge Motor Drivers
’H’-bridge motor drivers are used in a
wide range of products such as disc
d r i v e s , c o i n c o n t rol m e c han is ms ,
automotive applications, servo systems,
t o y s e tc . T h e se d r iv e r s p r o v i d e
bi-directional outputs from single polarity
supplies, usually under the control of a
logic IC or microcontroller. They are
usually constructed using two NPN and
two PNP transistors, all operating in
grounded emitter mode (see Figure 1). By
turning on one NPN device and the
diagonally opposite PNP device, (say
NPN1 and PNP1) virtually all the supply
voltage can be applied across the motor
load. Switching the second pair of
transistors instead reverses the supply to
the load. ’H’-bridge transistors often
require additional collector-emitter
diodes to protect the drivers from
regenerative currents and transients that
can be generated by the load motor.
AN12- 2
PNP1
+ve
DC MOTOR
NPN1
NPN2
-ve
Select
Line 1
Figure 1
Conceptual ’H’-Bridge Motor Driver.
In battery powered applications it is vital
that as much of the supply as possible is
applied across the load, maximising
battery life through greater efficiency and
lower end of life battery voltage. Using
the FMMT618 and FMMT718, the bridge
circuit shown in Figure 2 will handle
load/stall currents up to 1.5A. The circuit
can easily be adapted for lower current
motors by increasing the value of the
base drive resistors. (Set IB for the PNPs
to 1/50 of the maximum load current and
IB for the NPNs to 1/100). The saturation
voltage losses at 1.5A are a total of only
0.3V for both NPN & PNP transistors
combined, and at lower load currents less
than half this level can be expected.
The combination of low saturation losses
and low base drive requirements of the
FMMT618/718 gives improved motor
performance and endurance. Parallel
diodes are often not necessary for this
circuit as the reverse hFE of the driver
transistor is sufficiently high to conduct
regenerative currents and transients
safely away. The small size of the SOT23
package and reduced component count
allowed means this bridge circuit can be
constructed using much less PCB area
than previous designs, giving product
size and cost improvements.
For higher voltage motors, the controlling
logic is rarely capable of driving the PNP
transistors directly. In Figure 3, this problem
is overcome by the use of a pair of general
purpose NPN transistors to act as
buffers/level translators. The hFE of the
bridge transistors becomes more
important as the supply voltage to the
motor is increased as base drive losses are
supply dependant. The high hFE of the
FMMT618/718 means that these losses can
be minimised, saving PCB area and cost.
+15V
+4.8V
100
130
130
FMMT
718
100
FMMT
718
FMMT
718
BC
846
Controller
1.5A
1A
FMMT
718
BC
846
DC MOTOR
Controller
150
DC MOTOR
270
270
FMMT
618
FMMT
618
FMMT
618
FMMT
618
150
0V
0V
Figure 2
’H’-Bridge Motor Driver using SuperSOT
Bipolar Transistors.
Figure 3
’H’-Bridge Motor Driver with Small Signal
Buffer Transistors.
AN12 - 3
Application Note 12
Issue 2 January 1996
Application Note 12
Issue 2 January 1996
The SuperSOT Series
POLARITY
NPN
PNP2
PNP
FMMT618
FMMT619
FMMT624
FMMT625
FMMT718
FMMT720
FMMT722
FMMT723
BVCEO
20V
50V
125V
150V
20V
40V
70V
100V
IC CONT
2.5A
2A
1A
1A
1.5A
1.5A
1.5A
1A
IC MAX
6A
6A
3A
3A
6A
4A
3A
2.5A
Midband hFE
450
450
450
450
450
450
450
450
Typical hFE
at IC
360
2A
225
2A
140
1A
45
1A
230
2A
290
1A
275
1A
250
1A
Typical
VCE(sat)
at IC
130mV
150mV
165mV
180mV
145mV
245mV
140mV
210mV
2.5A
2A
1A
A
1.5A
1.5A
1A
1A
Ptot
625mW
625mW
625mW
625mW
625mW
625mW
625mW
625mW
*Measured with device mounted on a 15 x 15 x 0.6 mm ceramic substrate.
Table 1
NPN and PNP SuperSOT Series Parametric Overview.
These features combined with high
mid-band hFE and fast switching speeds
make the series ideal for switching
applications such as DC-DC converters,
motor drivers, lamp and solenoid drivers,
display drivers, power supply line
switching, buffers etc. These transistors
will also suit linear applications, e.g. the
low series base resistance inherent in the
S u pe r S O T des ign appr oach gives
excellent low noise performance.
A f e w e x a mp l e a p p li ca t i o n s a r e
suggested below, that exploit many of
the key features of the FMMT718/618
series.
’H’-Bridge Motor Drivers
’H’-bridge motor drivers are used in a
wide range of products such as disc
d r i v e s , c o i n c o n t rol m e c han is ms ,
automotive applications, servo systems,
t o y s e tc . T h e se d r iv e r s p r o v i d e
bi-directional outputs from single polarity
supplies, usually under the control of a
logic IC or microcontroller. They are
usually constructed using two NPN and
two PNP transistors, all operating in
grounded emitter mode (see Figure 1). By
turning on one NPN device and the
diagonally opposite PNP device, (say
NPN1 and PNP1) virtually all the supply
voltage can be applied across the motor
load. Switching the second pair of
transistors instead reverses the supply to
the load. ’H’-bridge transistors often
require additional collector-emitter
diodes to protect the drivers from
regenerative currents and transients that
can be generated by the load motor.
AN12- 2
PNP1
+ve
DC MOTOR
NPN1
NPN2
-ve
Select
Line 1
Figure 1
Conceptual ’H’-Bridge Motor Driver.
In battery powered applications it is vital
that as much of the supply as possible is
applied across the load, maximising
battery life through greater efficiency and
lower end of life battery voltage. Using
the FMMT618 and FMMT718, the bridge
circuit shown in Figure 2 will handle
load/stall currents up to 1.5A. The circuit
can easily be adapted for lower current
motors by increasing the value of the
base drive resistors. (Set IB for the PNPs
to 1/50 of the maximum load current and
IB for the NPNs to 1/100). The saturation
voltage losses at 1.5A are a total of only
0.3V for both NPN & PNP transistors
combined, and at lower load currents less
than half this level can be expected.
The combination of low saturation losses
and low base drive requirements of the
FMMT618/718 gives improved motor
performance and endurance. Parallel
diodes are often not necessary for this
circuit as the reverse hFE of the driver
transistor is sufficiently high to conduct
regenerative currents and transients
safely away. The small size of the SOT23
package and reduced component count
allowed means this bridge circuit can be
constructed using much less PCB area
than previous designs, giving product
size and cost improvements.
For higher voltage motors, the controlling
logic is rarely capable of driving the PNP
transistors directly. In Figure 3, this problem
is overcome by the use of a pair of general
purpose NPN transistors to act as
buffers/level translators. The hFE of the
bridge transistors becomes more
important as the supply voltage to the
motor is increased as base drive losses are
supply dependant. The high hFE of the
FMMT618/718 means that these losses can
be minimised, saving PCB area and cost.
+15V
+4.8V
100
130
130
FMMT
718
100
FMMT
718
FMMT
718
BC
846
Controller
1.5A
1A
FMMT
718
BC
846
DC MOTOR
Controller
150
DC MOTOR
270
270
FMMT
618
FMMT
618
FMMT
618
FMMT
618
150
0V
0V
Figure 2
’H’-Bridge Motor Driver using SuperSOT
Bipolar Transistors.
Figure 3
’H’-Bridge Motor Driver with Small Signal
Buffer Transistors.
AN12 - 3
Application Note 12
Issue 2 January 1996
Both of these ’H’-bridge circuits are
intended for direct logic drive though the
circuit given in Figure 2 requires logic
with high current outputs (up to 30mA) if
1.5A loads are to be driven. If this logic
drive level is not available, the buffer
circuit of Figure 3 provides a solution
which is not only inexpensive, but
outperforms most SOT89 solutions and
many SOT223 based circuits too.
which can be used to introduce a turn-on
de lay w i th ou t eff e c t ing tur n-off
performance - sometimes required to
avoid cross conduction problems in
push-pull output stages.
1K
FMMT
618
2K2
FMMT
2369
0*
IRF830
+12V
2K2
FMMT
718
22pF
FMMT
618
PWM Controller
Complimentary emitter followers as
shown in Figure 4 can provide an ideal
buffer function if transistors of high
current capability combined with high fT
are utilised. The FMMT618 and FMMT718
pr ov ide
th is
com bination
of
characteristics so that the 10nF effective
capacitance of two IRF840 MOSFETs in
parallel can be charged to 12V in under
30ns - a feat requiring a peak current of
around 4A. The circuit includes a resistor
+12V
+5V
PWM Controller
Power MOSFET Gate Drivers
The input capacitance of power MOSFETs
and IGBTs range from a few hundred
picofarads to tens of nanofarads. When
Miller effects are taken into account (the
amplification of feedback capacitances)
by using the more valid method of
evaluating gate charge rather than Ciss to
calculate effective input capacitance,
values around three times higher are
obtained. To minimise switching losses in
these power devices, particularly in high
frequency converters, it is vital that the
gate capacitances are charged and
discharged as rapidly as possible.
Consequently, driver circuits must act as
low impedance voltage sources, capable
of supplying large transient charge
currents. Since standard switching power
supply control ICs are rarely able to drive
larger capacitance MOSFETs adequately,
a high speed buffer is often used.
Application Note 12
Issue 2 January 1996
0V
( * Set turn-on delay )
0*
2X
IRF840
FMMT
718
0V
Figure 4
MOSFET/IGBT Complimentary Emitter
Follower Gate Driver.
Where 5V logic provides a custom pulse
width modulated controller, a buffer can
be required to level translate, giving 10V
or greater gate drive for the power
sw i tc hes . B y us ing an FMMT236 9
switching transistor, the circuit shown in
Figure 5 converts 5V logic drive to a 12V
gate drive signal. Driving the emitter of
the FMMT2369 from the logic output
avoids signal inversion.
Figure 5
MOSFET/IGBT Gate Driver using Emitter
Driven FMMT2369 for Buffering.
By giving excellent high current
performance in a SOT23 package, the
FMMT618 and FMMT718 replace SOT223
and SOT89 transistors in these gate drive
circuits leading to cost and PCB area
savings - particularly in very high
frequency converters.
’H’-Bridge Siren Driver
Many modern burglar and automotive
alarm sirens employ an 8Ω moving coil
lou d s pe ak e r d r i v e n b y a b i p o l a r
AN12- 4
A specially designed Zetex Siren Driver
IC, the ZSD100, provides a variable
frequency drive to the SOT23 ’H’-bridge
ensuring a very loud (and irritating!) noise
is generated. The combination of the
ZSD100 and a SOT23 ’H’-bridge produces
an extremely compact and inexpensive
module.
1N4000 +12V
Trim
Vin
FMMT
FMMT
720 8Ω 720
Op1
Enbl
The FMMT2369 gold doped switching
transistor has a very short storage time
which, combined with the high gain
FMMT618 gives the circuit a turn-on time
of only 20ns when driving a MOSFET with
an effective input capacitance of 2nF. The
FMMT718 helps make turn-off times even
s ho r ter ,
leading
to
r e duc ed
cross-conduction problems in bridge or
push-pull converters.
’H’-bridge. Handling peak output currents
of 2A, the TO126 or TO220 packaged
output transistors normally used require
parallel collector emitter diodes to divert
destructive reverse transients generated
by the inductive load. In the circuit shown
in Figure 6, FMMT619 and FMMT720
SOT23 transistors replace these bulky
and expensive leaded transistors, giving
other savings too. High reverse hFE,
inherent in the matrix technology used to
manufa cture the Zetex transistors
eliminates the need for parallel collector
emitter protection diodes. The FMMT619
and FMMT720 conduct reverse collector
current sufficiently well to clamp any
inductive transients generated by the
load.
ZSD100
C2
Cswp
Op2
Cosc
Gnd
150
150
SPEAKER
FMMT
619
C1
FMMT
2222A
FMMT
619
FMMT
2222A
0V
Figure 6
’H’-Bridge for Driving Moving Coil Loudspeakers within Alarm/Siren Systems.
AN12 - 5
Application Note 12
Issue 2 January 1996
Both of these ’H’-bridge circuits are
intended for direct logic drive though the
circuit given in Figure 2 requires logic
with high current outputs (up to 30mA) if
1.5A loads are to be driven. If this logic
drive level is not available, the buffer
circuit of Figure 3 provides a solution
which is not only inexpensive, but
outperforms most SOT89 solutions and
many SOT223 based circuits too.
which can be used to introduce a turn-on
de lay w i th ou t eff e c t ing tur n-off
performance - sometimes required to
avoid cross conduction problems in
push-pull output stages.
1K
FMMT
618
2K2
FMMT
2369
0*
IRF830
+12V
2K2
FMMT
718
22pF
FMMT
618
PWM Controller
Complimentary emitter followers as
shown in Figure 4 can provide an ideal
buffer function if transistors of high
current capability combined with high fT
are utilised. The FMMT618 and FMMT718
pr ov ide
th is
com bination
of
characteristics so that the 10nF effective
capacitance of two IRF840 MOSFETs in
parallel can be charged to 12V in under
30ns - a feat requiring a peak current of
around 4A. The circuit includes a resistor
+12V
+5V
PWM Controller
Power MOSFET Gate Drivers
The input capacitance of power MOSFETs
and IGBTs range from a few hundred
picofarads to tens of nanofarads. When
Miller effects are taken into account (the
amplification of feedback capacitances)
by using the more valid method of
evaluating gate charge rather than Ciss to
calculate effective input capacitance,
values around three times higher are
obtained. To minimise switching losses in
these power devices, particularly in high
frequency converters, it is vital that the
gate capacitances are charged and
discharged as rapidly as possible.
Consequently, driver circuits must act as
low impedance voltage sources, capable
of supplying large transient charge
currents. Since standard switching power
supply control ICs are rarely able to drive
larger capacitance MOSFETs adequately,
a high speed buffer is often used.
Application Note 12
Issue 2 January 1996
0V
( * Set turn-on delay )
0*
2X
IRF840
FMMT
718
0V
Figure 4
MOSFET/IGBT Complimentary Emitter
Follower Gate Driver.
Where 5V logic provides a custom pulse
width modulated controller, a buffer can
be required to level translate, giving 10V
or greater gate drive for the power
sw i tc hes . B y us ing an FMMT236 9
switching transistor, the circuit shown in
Figure 5 converts 5V logic drive to a 12V
gate drive signal. Driving the emitter of
the FMMT2369 from the logic output
avoids signal inversion.
Figure 5
MOSFET/IGBT Gate Driver using Emitter
Driven FMMT2369 for Buffering.
By giving excellent high current
performance in a SOT23 package, the
FMMT618 and FMMT718 replace SOT223
and SOT89 transistors in these gate drive
circuits leading to cost and PCB area
savings - particularly in very high
frequency converters.
’H’-Bridge Siren Driver
Many modern burglar and automotive
alarm sirens employ an 8Ω moving coil
lou d s pe ak e r d r i v e n b y a b i p o l a r
AN12- 4
A specially designed Zetex Siren Driver
IC, the ZSD100, provides a variable
frequency drive to the SOT23 ’H’-bridge
ensuring a very loud (and irritating!) noise
is generated. The combination of the
ZSD100 and a SOT23 ’H’-bridge produces
an extremely compact and inexpensive
module.
1N4000 +12V
Trim
Vin
FMMT
FMMT
720 8Ω 720
Op1
Enbl
The FMMT2369 gold doped switching
transistor has a very short storage time
which, combined with the high gain
FMMT618 gives the circuit a turn-on time
of only 20ns when driving a MOSFET with
an effective input capacitance of 2nF. The
FMMT718 helps make turn-off times even
s ho r ter ,
leading
to
r e duc ed
cross-conduction problems in bridge or
push-pull converters.
’H’-bridge. Handling peak output currents
of 2A, the TO126 or TO220 packaged
output transistors normally used require
parallel collector emitter diodes to divert
destructive reverse transients generated
by the inductive load. In the circuit shown
in Figure 6, FMMT619 and FMMT720
SOT23 transistors replace these bulky
and expensive leaded transistors, giving
other savings too. High reverse hFE,
inherent in the matrix technology used to
manufa cture the Zetex transistors
eliminates the need for parallel collector
emitter protection diodes. The FMMT619
and FMMT720 conduct reverse collector
current sufficiently well to clamp any
inductive transients generated by the
load.
ZSD100
C2
Cswp
Op2
Cosc
Gnd
150
150
SPEAKER
FMMT
619
C1
FMMT
2222A
FMMT
619
FMMT
2222A
0V
Figure 6
’H’-Bridge for Driving Moving Coil Loudspeakers within Alarm/Siren Systems.
AN12 - 5
Application Note 12
Issue 2 January 1996
Application Note 12
Issue 2 January 1996
FMMT
718
220uH
+5V/
+3.3V
30K
220
150
LL5818
470uF
0.05
ILim
Vin
LM3578A
+ve
Gnd
1000uF
C
-ve
Vin
E
22pF
470K
Osc
1.8nF
0V
7K5/
13K
220pF (Set Vout)
0V
Figure 7
DC-DC Step-down Converter Effected with PNP SuperSOT. High Current Paths Shown
in Bold.
Figure 9
Efficiency Vs Input Voltage Profile for FMMT718 DC-DC Converter of Figure 7.
DC-DC Converter
Using standard PWM controllers it is easy
to construct Buck step down converters
with low component counts. Harder to
achieve are designs that are both simple
and efficient as required for today’s
battery operated equipment. The key to
maximising efficiency is eliminating
voltage drops in all high current areas.
Figure 8
Efficiency Vs Output Current Profile for FMMT718 DC-DC Converter of Figure 7.
AN12- 6
In the Buck converter shown in Figure 7,
the high current paths are via the 50mΩ
sense resistor, the series switching
transistor, output inductor L1 and the
Schottky diode. Once the resistance of the
output inductor has been minimised, the
most critical component (particularly
w he n VIN approaches VOUT) i s the
saturation voltage drop of the switching
transistor. By using an FMMT718, which
drops only 200mV @ 1.5A, this converter
can operate at an efficiency of over 90%
at minimum input voltage and an IOUT of
1.5A. As the input voltage is increased,
the operating gain of the switching
transistor becomes more important. The
high gain of this transistor allows base
drive losses to be minimised leading to
high efficiencies over a wide supply
range. (Please refer to Figures 8 and 9).
The fast rise and fall times of the
FMMT718 allows the converter to operate
at 50kHz with minimal switching losses.
At this frequency it is essential to use low
ESR input and output capacitors, and
keep any wires carrying switched high
currents very short so as to minimise RFI
and output ripple. (These wires are shown
as bold in Figure 7).
The converter will operate from a supply
of (VOUT +0.5V) up to 16V. The converter’s
output voltage can be set to 5V or 3.3V by
selection of R1. The circuit will supply
loads from 0 to 1.5A, current limiting to
around 2A with a shorted output.
AN12 -7
Application Note 12
Issue 2 January 1996
Application Note 12
Issue 2 January 1996
FMMT
718
220uH
+5V/
+3.3V
30K
220
150
LL5818
470uF
0.05
ILim
Vin
LM3578A
+ve
Gnd
1000uF
C
-ve
Vin
E
22pF
470K
Osc
1.8nF
0V
7K5/
13K
220pF (Set Vout)
0V
Figure 7
DC-DC Step-down Converter Effected with PNP SuperSOT. High Current Paths Shown
in Bold.
Figure 9
Efficiency Vs Input Voltage Profile for FMMT718 DC-DC Converter of Figure 7.
DC-DC Converter
Using standard PWM controllers it is easy
to construct Buck step down converters
with low component counts. Harder to
achieve are designs that are both simple
and efficient as required for today’s
battery operated equipment. The key to
maximising efficiency is eliminating
voltage drops in all high current areas.
Figure 8
Efficiency Vs Output Current Profile for FMMT718 DC-DC Converter of Figure 7.
AN12- 6
In the Buck converter shown in Figure 7,
the high current paths are via the 50mΩ
sense resistor, the series switching
transistor, output inductor L1 and the
Schottky diode. Once the resistance of the
output inductor has been minimised, the
most critical component (particularly
w he n VIN approaches VOUT) i s the
saturation voltage drop of the switching
transistor. By using an FMMT718, which
drops only 200mV @ 1.5A, this converter
can operate at an efficiency of over 90%
at minimum input voltage and an IOUT of
1.5A. As the input voltage is increased,
the operating gain of the switching
transistor becomes more important. The
high gain of this transistor allows base
drive losses to be minimised leading to
high efficiencies over a wide supply
range. (Please refer to Figures 8 and 9).
The fast rise and fall times of the
FMMT718 allows the converter to operate
at 50kHz with minimal switching losses.
At this frequency it is essential to use low
ESR input and output capacitors, and
keep any wires carrying switched high
currents very short so as to minimise RFI
and output ripple. (These wires are shown
as bold in Figure 7).
The converter will operate from a supply
of (VOUT +0.5V) up to 16V. The converter’s
output voltage can be set to 5V or 3.3V by
selection of R1. The circuit will supply
loads from 0 to 1.5A, current limiting to
around 2A with a shorted output.
AN12 -7
Application Note 12
Issue 2 January 1996
LCD Backlighting Converter
Cold cathode fluorescent lamps as used
for portable computer LCD backlighting
and similar applications, require a
converter generating between 1 and 2kV
to strike and run. Standard circuits
provide control of tube brightness against
input supply variations, and other factors
such as temperature, tube ageing etc.
These circuits commonly use SOT223
transistors in the high voltage converter,
since high currents must be passed with
minimal saturation losses if good
efficiency is to be achieved. In Figure 10,
FMMT619 SOT23 transistors have been
used to replace SOT223 BCP56 types. The
FMMT619s, exhibiting a saturation
voltage of only 125mV at 1A - less than
half that of the BCP56s, not only reduces
cost and PCB area, but also raises
converter efficiency over the original
SOT223 transistors.
CCFT
15pF
3KV
9
7
W4
CTX110092-1
2
+5 to 15V
W1
W2
4
3
Vin
22nF
1K
W3
FMMT
619
5
FMMT
619
1
10uF
LL5818
CTX300-4
300uH
Vin
NC
1/2
Vsw
E1
BAV99
LTC1172
Vfb
E2
Gnd
10K
Vc
1uF
0V
2.2uF
Gnd
Figure 10
LCD Backlighting Inverter.
AN12- 8
50K
560
1/2
BAV99
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